Downhole cleaning apparatus

ABSTRACT

An apparatus for cleaning the area around a casing of a wellbore is described. The apparatus comprises a body configured to be located in a wellbore casing, the body defining an internal chamber for receipt of pressurised fluid. Pistons are mounted in the body and is arranged to move from an inwardly retracted condition to an outwardly deployed condition as a result of an increase in fluid pressure in the internal chamber. Pistons further comprise at least one first nozzle arranged to direct a jet of pressurised fluid from the apparatus.

A downhole cleaning apparatus for cleaning the area around a casing of a wellbore is disclosed. This relates particularly, but not exclusively to an apparatus for cleaning the annulus outside of a wellbore casing. A method of cleaning the area around a wellbore casing is also disclosed.

Wellbores for the production of hydrocarbons are generally lined with steel casing to amongst other things prevent the sides of the wellbore from collapsing. Several lengths of casing of different diameters may concentrically overlap in a wellbore. In many situations, there is an outer annulus in the form of a gap between the outside of an inner casing and the inside of an outer casing or surface of the wellbore. This annulus can fill with debris such as cement, barite and other solids.

It can be desirable to clean this annulus and other parts of the casing to for example enable cementing in plug and abandonment operations where the wellbore is to be decommissioned and sealed. Known methods of cleaning the annulus can be costly and it can be difficult to select particular areas of the annulus to clean.

WO2009/128915 discloses a device for cleaning a well casing. The device comprises a tubular body on which moveable brush assemblies and nozzles are disposed. The nozzles are biased outwardly under pressure but are designed to retract when contact is made with the well casing to prevent damage to the nozzles. The device is designed to be raised and lowered up and down a well casing to cause reciprocation of the brushes on the internal surface of the casing.

This apparatus suffers from the drawback that it must be raised and lowered to clean. Also, the only contact made with the casing is by the brushes such that only the interior surface of the casing is readily cleaned.

Preferred embodiments seek to overcome the above disadvantages of the prior art.

An apparatus for cleaning the area around a wellbore casing is disclosed, the apparatus comprising:

a body configured to be located in a casing disposed in a wellbore, the body defining an internal chamber for receipt of pressurised fluid; at least one piston mounted to the body and being moveable from an inwardly retracted condition to the outwardly deployed condition as a result of an increase in fluid pressure in the internal chamber, such that a predetermined pressure differential between the internal chamber and the outside of the apparatus moves said at least one piston to the outwardly deployed condition; and wherein said at least one piston further comprises an impact surface arranged to slidably engage the internal surface of a perforated casing in use and impart vibrations to said casing when the apparatus is rotated in the wellbore and the impact surface slides along said internal surface of said perforated 30 casing.

This provides the advantage of an apparatus that is capable of cleaning the area around a wellbore casing by imparting vibrations to the casing. In many circumstances, a wellbore casing is perforated which forms a profile around the inner casing circumference. By rotating the apparatus the contact members will move up and down as the impact surfaces slide along the profile in a percussive, hammer-like action which sets up a vibration in the casing to dislodge cement and other material.

This therefore provides the advantage of an apparatus capable of cleaning the outer diameter a wellbore casing. A casing may have been cemented many years previously and impacting the inside of the casing helps to break old cement from the outside of the casing, as well as from the wellbore around the casing.

Furthermore, the apparatus provides the advantage that it can be used during operations to cement the casing. Agitation is commonly used in civil engineering applications to improve the placement of cement. Consequently, rotating the apparatus in a perforated casing can be used to agitate the casing during a re-cementing operation. This helps cement to flow to hard to reach places and is therefore particularly advantageous to create an effective cement barrier to prevent hydrocarbon migration for example if the cased wellbore is to be abandoned.

This also provides the advantage of an apparatus that can be located at a chosen point in a wellbore, for example an area where the casing is perforated, and actuated by increasing fluid pressure in the apparatus to deploy the pistons. This is an extremely straightforward method of operating a cleaning apparatus. This also provides the advantage that the cleaning apparatus is reusable and can be moved along the wellbore to different points in a single cleaning run.

In a preferred embodiment, the apparatus further comprises at least one nozzle arranged to direct a jet of pressurised fluid from the internal chamber out of the apparatus.

This provides the advantage of an apparatus that is capable of cleaning the area around a wellbore casing by both use of pressurised fluid and by imparting vibrations to the casing. The jets of fluid move against the casing and through perforations to both clean and carry debris to the surface.

This therefore provides the advantage of an apparatus capable of cleaning the outer diameter a wellbore casing. A casing may have been cemented many years previously and impacting the inside of the casing helps to break old cement from the outside of the casing, as well as from the wellbore around the casing. The jetted fluid can then wash the released broken down cement away.

Said at least one nozzle may be formed in said piston.

This provides the advantage that the direction of the nozzles can be chosen to be directed through various orientations of wellbore perforation to ensure good cleaning.

By directing the nozzles through the perforations in the wellbore, this creates a flow of fluid which will carry 30 debris up to the surface. The high pressure jets also assist in dislodging materials and debris from the casing and annulus.

Said piston may comprise an aperture through which a retaining bar projects, said retaining bar being mounted to the body to retain the piston in the body.

This provides the advantage that the pistons can freely move in and out of the body as the apparatus is rotated and the pistons contact different parts of the profile of the perforated casing. For example, as a piston is moved along the circular outer surface of the casing to a point at which it encounters a perforated part of the casing, the piston is able to move further outwardly from the body to hammer against the perforated part of the casing. Such a hammer-like action causes vibration in the casing to assist in the dislodgement of cement and other materials.

In a preferred embodiment, the apparatus further comprises a mandrel disposed in said internal chamber, the mandrel comprising at least one port, the mandrel being moveable along the internal chamber from a position in which said at least one port is blocked to a position enabling fluid to be pumped through said at least one port to move said piston to the outwardly deployed condition.

This provides the advantage of a straightforward means of maintaining the pistons in the inwardly retracted condition until such time as they are required to be deployed.

In a preferred embodiment, the mandrel comprises a restriction adapted to receive a first ball or dart dropped through the apparatus to block fluid flow through the mandrel and enable fluid pressure to increase to move the mandrel to a position enabling fluid to be pumped through said at least one port.

Said ball or dart may be deformable.

This provides the advantage of enabling the apparatus to be reset and the mandrel cleared to recommence fluid flow through a work string in which the apparatus is disposed. This is advantageous because it allows the work string to perform an alternative function.

In a preferred embodiment, said at least one port is configured to be blocked by a second ball to enable fluid pressure to increase to force said first ball or dart through said restriction to cause retraction of said at least one piston.

This provides the advantage that the apparatus can be reset to provide a multi-function tool that can be deployed numerous times in a single downhole run to clean various different sections of wellbore.

The apparatus may further comprise at least one shear pin arranged to retain said mandrel in the position in which said at least one port is blocked.

The apparatus may further comprise at least one second nozzle formed through the body.

This provides the advantage of a further cleaning function of the apparatus.

The apparatus may further comprise a plurality of pistons.

Said impact surface may be curved.

This provides the advantage of facilitating sliding of the impact surface along the internal surface of the casing.

A method of cleaning the area around a wellbore casing is also disclosed, the method comprising:

locating an apparatus as defined above in a casing disposed in a wellbore; increasing fluid pressure in the internal chamber to move said at least one piston to the outwardly deployed condition; and rotating the apparatus.

This provides the advantage of a method of cleaning a wellbore which both enables high pressure fluid jets to be used to dislodge cement and other materials and also impart vibrations to the casing to assist in the dislodging of debris.

This also provides the advantage of a cleaning method which 25 is very straightforward to carry out as all that is required is to increase fluid pressure in a work string in which the apparatus is located and then rotate the part of the work string at which the apparatus is located.

The step of locating the apparatus in the casing may include locating the apparatus at a position in the casing at which the casing is perforated.

The step of increasing fluid pressure in the internal chamber may produce a jet of fluid from said at least one nozzle.

Furthermore, an apparatus for cleaning the area around a wellbore casing is disclosed, the apparatus comprising:

a body configured to be located in a casing disposed in a wellbore, the body defining an internal chamber for receipt of pressurised fluid; at least one contact member mounted to the body and being moveable in and out of the body, wherein said at least one contact member is urged by spring means towards an outwardly deployed condition to engage the casing, said at least one contact member further comprising an impact surface arranged to slidably engage the internal surface of a perforated casing in use and impart vibrations to said casing when the apparatus is rotated in the wellbore and the impact surface slides along said internal surface of said perforated casing; and at least one nozzle arranged to direct a jet of pressurised fluid from the internal chamber out of the apparatus.

This provides the advantage of an apparatus that is capable of cleaning the area around a wellbore casing by both use of pressurised fluid and by imparting vibrations to the casing. In many circumstances, a wellbore casing is perforated which forms a profile around the inner casing circumference. By rotating the apparatus the contact members will move up and down as the impact surfaces slide along the profile in a percussive, hammer-like action which sets up a vibration in the casing to dislodge cement and other material. The jets of fluid move against the casing and through perforations to both clean and carry debris to the surface.

This therefore provides the advantage of an apparatus capable of cleaning the outer diameter a wellbore casing. A casing may have been cemented many years previously and impacting the inside of the casing helps to break old cement from the outside of the casing, as well as from the wellbore around the casing. The jetted fluid can then wash the released broken down cement away.

Furthermore, the apparatus provides the advantage that it can be used during operations to cement the casing. Agitation is commonly used in civil engineering applications to improve the placement of cement. Consequently, rotating the apparatus in a perforated casing can be used to agitate the casing during a re-cementing operation. This helps cement to flow to hard to reach places and is therefore particularly advantageous to create an effective cement barrier to prevent hydrocarbon migration for example if the cased wellbore is to be abandoned.

Preferred embodiments will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of a first embodiment of an apparatus for cleaning the area around a casing of a wellbore, the apparatus being shown with the pistons in the inwardly retracted condition;

FIG. 2 is a cross-sectional view corresponding to FIG. 1 showing the pistons in the outwardly deployed condition;

FIG. 3 is a cross-sectional view taken along line c-c of FIG. 2;

FIG. 4 is a perspective view of the apparatus of FIGS. 1 to 3;

FIG. 5a is a cross-sectional view corresponding to FIG. 2 showing the fluid flow path;

FIG. 5b is a perspective view corresponding to FIG. 4 showing the fluid flow path and direction of jets issued from the nozzles;

FIG. 6 is a cross-sectional view of the apparatus of FIG. 1 located inside a perforated wellbore casing surrounded by an outer casing;

FIG. 7 is an end on view of two concentric wellbore 20 casings;

FIG. 8 is a side view of a perforated wellbore casing;

FIG. 9a is a longitudinal cross-sectional view of a second embodiment of an apparatus for cleaning the area around a casing of a wellbore, the apparatus being shown with the pistons in the inwardly retracted condition;

FIG. 9b is a cross-sectional view of the apparatus of 30 FIG. 9a showing the pistons in the outwardly deployed condition with a deformable ball seated in the ball seat;

FIG. 9c is a cross-sectional view of the apparatus of FIGS. 9a and 9b showing the first stage of resetting the tool with rigid balls blocking the ports;

FIG. 9d is a cross-sectional view of the apparatus of FIG. 9a in the reset condition showing the balls in the ball catcher;

FIG. 9e is a cross-sectional view of the apparatus of FIG. 9a showing an additional ball in the through ball section; and

FIG. 10 is a longitudinal cross-sectional view of a third embodiment of an apparatus for cleaning the area around a casing of a wellbore, the apparatus being shown with the contact members in the inwardly retracted condition.

Referring to FIGS. 1 to 6, a first embodiment of an apparatus 2 for cleaning the area around a casing of a wellbore comprises a body 4 configured to be located in a casing 8, the body defining an internal chamber 6 for receipt of pressurised fluid. Contact members 9 in the form of pistons 10 are mounted in the body and is arranged to move from an inwardly retracted condition as shown in FIG. 1 to an outwardly deployed condition as shown in FIGS. 2, 3, 4, 5 and 6 as a result of an increase in fluid pressure in the internal chamber 6. The pistons 10 further comprise nozzles 12 arranged to direct jets 14 of pressurised fluid from the apparatus 2.

In the example shown, the apparatus comprises three pairs of pistons 10 disposed in an equidistant fashion around the circumference of the apparatus 2. Each pair of pistons 10 is therefore located at a separation of 120° from the other pistons 10. Each contact member 9 further comprises an impact surface 11 arranged to slidably engage the internal surface of a perforated casing 8 and impart vibrations to the casing when the apparatus 2 is rotated in the casing such that the impact surfaces 11 slide along the internal surface of the casing 8. Preferably, impact surfaces are curved to facilitate sliding contact with casing 8. Impact surfaces may be formed from metallic and/or hardened material to prevent breakage.

Referring to FIGS. 1 to 3, each piston 10 comprises an aperture 16 through which a retaining bar 18 passes to retain the pistons 10 in the body. Pistons 10 can move freely up and down in the body to the extent to which the retaining bar 18 located in apertures 16 allows. As a consequence, an increase in fluid pressure in piston chamber 28 tends to bias the pistons 10 outwardly to the extent to which the retaining bar 18 allows. Piston nozzles 10 provide fluid communication between piston chamber 28, which is a continuation of internal chamber 6, and the outside of the apparatus 2. A mandrel 20 is located in internal chamber 6 along the longitudinal axis x-x of the body. The mandrel is moveable along axis x-x and comprises at least one port to enable fluid to pass out of the mandrel 20. The mandrel also comprises a restriction 24 at its lowermost end.

In the condition shown in FIG. 1, port 22 of mandrel 20 is blocked such that fluid flowing through internal chamber 6 and therefore mandrel 20 will simply pass through the apparatus 2. However, when a dart 26 is dropped down the apparatus, i.e. dropped from the surface through a work string (not shown) in which the apparatus is mounted as will be familiar to persons skilled in the art, the dart 26 has a diameter larger than that of restriction 24 such that the dart 26 will be caught by restriction 24 to block the lower end of mandrel 20 and prevent fluid flowing past mandrel 20. Dart 26 may be deformable to enable the mandrel to be cleared to recommence fluid flow through the apparatus 2. A deformable ball could also be used.

In this condition, when fluid is pumped through the apparatus 2, the fluid pressure will increase to a point at which shear pins (not shown) holding the mandrel in position rupture. A further increase in fluid pressure then pushes mandrel downwardly from the position shown in FIG. 1 to that of FIG. 2 which moves port 22 into the area of piston chamber 28 which causes piston chamber 28 to fill with pressurised fluid. At a pre-determined fluid pressure, pistons 10 move outwardly as jets of fluid 14 issue through piston nozzles 12 and nozzles 30 disposed on the body of the apparatus.

Referring to FIG. 7, a concentric arrangement of casings common in many wellbores is shown. Inner casing 8 defines what is known as the inner annulus “A”. An outer casing 32 also disposed in wellbore 34 defines outer annulus “B” between the inner casing 8 and outer casing 32. The “B” annulus may be filled with cement 36.

Referring to FIG. 8, in many circumstances, it is desirable to perforate inner casing 8 with a plurality of perforations 36 which are holes punched through the casing 8. Various methods to do this are used, but the perforations shown in FIG. 8 are of the type created by the perforating tool disclosed in WO2012/098377A2. This apparatus comprises hydraulically activated cutting elements which punch perforations 36 into a casing. The profile of the perforated casing of FIG. 8 is shown in cross section in FIG. 6. It can be seen that the profile consists of both circular sections 8 a and expanded deformed sections 8 b. The perforations 36 formed by the perforating tool of WO2012/09837782 are orientated circumferentially. As a consequence, piston nozzles 12 can be machined such that jets 14 direct fluid in a circumferential direction around the casing 8 which has been found to greatly increase the cleaning ability of the apparatus 2.

The operation of apparatus 2 to clean a wellbore casing and annuluses will now be described with reference to FIGS. 1 to 8.

The apparatus 2 is moved to a point in a casing 8 at which perforations 36 are formed. Dart 26 is then dropped down the work string containing the apparatus 2. The dart 26 lodges in restriction 24 of mandrel 20. Fluid is then pumped through the work string to apparatus 2. After a predetermined pressure is reached, parting pins (not shown) shear and mandrel 20 is able to move downwardly to align port 22 with piston chamber 28. A fluid flow path 38 (FIG. 5a ) is therefore set up to enable pressurised fluid to fill piston chamber 28 thereby pushing pistons 10 outwardly and causing jets 14 and 38 to issue from piston nozzles 12 and body nozzles 30. The pistons therefore move to the outwardly deployed condition as a result of a pressure differential between piston chamber 28 and the outside of apparatus 2.

Whilst continuing to pump fluid through the apparatus 2, the apparatus is then rotated using a mud motor or by simply rotating the whole work string from the surface, which causes impact surfaces 11 of the outwardly deployed contact members 9 in the form of pistons 10 to slide against the inner profile of casing 8. Referring to FIG. 6, it can be seen that the pistons 10 move in and out of the body 4 depending on their position in inner casing 8. The nozzles 12 are formed in pistons 10 at an orientation to enable jets 14 to spray through of perforations 36. However, the location of the nozzles 12 can be chosen to suit any type of casing perforation rather than that shown in the drawings.

As the apparatus 2 is rotated, a contact member 9 sliding along circular section 88 will pop out into a perforated section 8 b causing impact surface 11 to hammer against the casing 8. Continued rotation and the hammering action of the impact surfaces 11 sets up a vibration in casing 8 which dislodges debris such as cement which can be carried to the surface as a result of fluid flow set up by jets 14. Jets 14 and 30 also assist in the cleaning action by removing debris lodged on the casings 8 and 32 by the impact of the fluid jets. The action of impact surfaces 11 on the inside of the casing 8 therefore helps to break old cement from the outside of the casing, as well as from the wellbore around the casing. The jetted fluid can then wash the released broken down cement away.

It should be noted that any kind of blocking device such as a ball or dart can be used. In an alternative embodiment, the mandrel 20 can be removed completely and the pistons 10 deployed by a simple increase in fluid pressure.

A downhole apparatus 2 that is able to use oscillatory impact loading and vibration in order to dislodge cement, barite, and other solids in the B annulus of an oil and gas well is therefore disclosed. In addition to the impact vibration, jetted fluid is used to wash and carry debris up hole back to surface for a full and thorough clean of the A and B annuluses. The jets are specifically tailored to the type of perforation or mechanical casing cut in order to optimize the fluid trajectory and velocity at the boundary point.

Apparatus 2 can also be used during operations to cement the casing 8. Agitation is commonly used in civil engineering applications to improve the placement of cement. Consequently, rotating the apparatus 2 in a perforated casing 8 can be used to agitate the casing during a re-cementing operation. This helps cement to flow to hard to reach places and is therefore particularly advantageous to create an effective cement barrier to prevent hydrocarbon migration for example if the cased wellbore is to be abandoned.

Referring to FIGS. 9a to 9e , a second embodiment of an apparatus 102 for cleaning the area around a casing of a wellbore is shown with parts common to the embodiment of FIGS. 1 to 6 denoted by like reference numerals but increased by 100.

Apparatus 102 for cleaning the area around a wellbore casing comprises a body 104 configured to be located in a casing disposed in a wellbore. The body 104 defines an internal chamber 106 for receipt of a pressurised fluid. At least one piston member 110 is mounted to the body and is moveable from an inwardly contracted condition (FIG. 9a ) to outwardly deployed condition (FIG. 9b ) as a result of an increase in fluid pressure in chamber 106. A predetermined pressure differential between the internal chamber 106 and the outside of the apparatus biases pistons 110 into the outwardly deployed condition. Pistons 110 comprise impact surfaces 111 arranged to slide against the surface of a perforated wellbore casing to impart vibrations in the same manner as that of the embodiment of FIGS. 1 to 6. Nozzles are also provided to jet fluid in the same manner as that of the embodiment of FIGS. 1 to 6.

In this embodiment, mandrel 120 is biased by spring 121 into the deactivated position as shown in FIG. 9a . In this position, ports 122 of mandrel 120 are blocked and are not in fluid communication with piston chamber 128. A retaining bar 118 is disposed in piston chamber 128 to retain pistons 110.

In order to activate the pistons 110, a first deformable ball 126 is dropped into the string in which the apparatus 102 is located. The ball 126 falls until it hits restriction 124 and seats therein. This causes an increase in fluid pressure on pumping above the ball 126. Once the fluid pressure reaches a predetermined value, the mandrel 120 is forced downwardly overcoming the resistance of spring 121 until ports 122 align with piston chamber 128 to allow pressurised fluid to flow into the piston chamber 128 and push pistons 110 outwardly.

In order to deactivate the apparatus, two rigid balls such as steel balls 127 are dropped. The rigid balls 127 fall until they hit deformable ball 126 and seat in ports 122. This blocks ports 122 to cause a further pressure increase since the pumped fluid cannot flow into piston chamber 128. As a consequence, at a second predetermined pressure, deformable ball 126 deforms to the extent that it is pushed through restriction 122. Rigid balls 127 are dimensioned to be smaller than restriction 122 such that they also fall through the restriction. This reduces fluid pressure such that spring 121 can push mandrel back to the deactivated position as shown in FIG. 9d . The fluid pressure in piston chamber 128 is now insufficient to maintain deployment of pistons 110 which are caused to retract. The deformable 126 and rigid 127 balls fall until they are caught by a ball catcher assembly 140.

FIG. 9e shows the ball catcher 140 holding a further ball 129. Ball catcher 140 allows multiple ball activated tools to be run in the same work string. For example, a first tool would use slightly larger balls that would be retained in the ball catcher sleeve. A second lower tool would use slightly smaller balls 129 which would exit windows 142 in the ball catcher sleeve and travel down into the lower tool activating and deactivating as required. This enables a work string to have two or more ball activated tools.

As an example of the operation of the apparatus 102 of FIGS. 9a to 9e , the deformable balls 126 might be formed from plastic and have a diameter of 1.5 inches. The mandrel ports 122 are 1 inch wide and the steel deactivation balls 127 are 1.25 inches wide. The ball seat restriction 124 may be 1.45 inches wide in this embodiment such that the deformable ball 126 must deform by 0.05 inches. The system is set up such that this happens at approximately 3000 psi.

Referring to FIG. 10, a third embodiment of an apparatus for cleaning the area around a casing of a wellbore is shown with parts common to the embodiment of FIGS. 1 to 6 denoted by like reference numerals but increased by 200.

Apparatus 202 comprises a body 204 configured to be located in a casing disposed in a wellbore. The body 204 defines an internal chamber 206 for receipt of pressurised fluid. At least one contact member 209 is mounted to the body 204 and is moveable in and out of the body. The contact members 209 are biased towards the outwardly deployed condition by springs 211. Nozzles 213 provide fluid communication between internal chamber 206 and the outside of the apparatus 202 to enable pressurised fluid to be sprayed against the casing of a wellbore.

The embodiment of FIG. 9 is therefore operated by locating the apparatus 202 at a desired point in a wellbore, increasing fluid pressure in internal chamber 206 to spray jets of fluid from nozzles 213 whilst rotating the apparatus 202 which causes contact members 209 to engage the inner profile of a casing and cause vibration in a similar manner to that described in relation to the embodiment of FIGS. 1 to 6.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of protection as defined by the appended claims. 

1. An apparatus for cleaning the area around a wellbore casing, the apparatus comprising: a body configured to be located in a casing disposed in a wellbore, the body defining an internal chamber for receipt of pressurised fluid; at least one piston mounted to the body and being moveable from an inwardly retracted condition to the outwardly deployed condition as a result of an increase in fluid pressure in the internal chamber, such that a predetermined pressure differential between the internal chamber and the outside of the apparatus moves said at least one piston to the outwardly deployed condition; and wherein said at least one piston further comprises an impact surface arranged to slidably engage the internal surface of a perforated casing in use and impart vibrations to said casing when the apparatus is rotated in the wellbore and the impact surface slides along said internal surface of said perforated casing.
 2. An apparatus according to claim 1, further comprising at least one nozzle arranged to direct a jet of pressurised fluid from the internal chamber out of the apparatus.
 3. An apparatus according to claim 2, wherein said at least one nozzle is formed in said piston.
 4. An apparatus according to claim 1, wherein said piston comprises an aperture through which a retaining bar projects, said retaining bar mounted to the body to retain the piston in the body.
 5. An apparatus according to claim 1, further comprising a mandrel disposed in said internal chamber, the mandrel comprising at least one port, the mandrel being moveable along the internal chamber from a position in which said at least one port is blocked to a position enabling fluid to be pumped through said at least one port to move said piston to the outwardly deployed condition.
 6. An apparatus according to claim 5, wherein the mandrel comprises a restriction adapted to receive a first ball or dart dropped through the apparatus to block fluid flow through the mandrel and enable fluid pressure to increase to move the mandrel to a position enabling fluid to be pumped through said at least one port.
 7. An apparatus according to claim 6, wherein said first ball or dart is deformable.
 8. An apparatus according to claim 5, wherein said at least one port is configured to be blocked by a second ball to enable fluid pressure to increase to force said first ball or dart through said restriction to cause retraction of said at least one piston.
 9. An apparatus according to any one of claim 5, further comprising at least one shear pin arranged to retain said mandrel in the position in which said at least one port is blocked.
 10. An apparatus according to claim 1, further comprising at least one second nozzle formed through the body.
 11. An apparatus according to claim 1, further comprising a plurality of pistons.
 12. An apparatus according to claim 1, wherein said impact surface is curved.
 13. A method of cleaning the area around a wellbore casing, the method comprising: locating an apparatus according to claim 1 in a casing disposed in a wellbore; increasing fluid pressure in the internal chamber to move said at least one piston to the outwardly deployed condition; and rotating the apparatus.
 14. A method according to claim 13, wherein the step of locating the apparatus in the casing includes locating the apparatus at a position in the casing at which the casing is perforated.
 15. A method according to claim 13, wherein the step of increasing fluid pressure in the internal chamber produces a jet of fluid from said at least one nozzle.
 16. An apparatus for cleaning the area around a wellbore casing, the apparatus comprising: a body configured to be located in a casing disposed in a wellbore, the body defining an internal chamber for receipt of pressurised fluid; at least one contact member mounted to the body and being moveable in and out of the body, wherein said at least one contact member is urged by spring means towards an outwardly deployed condition to engage the casing, said at least one contact member further comprising an impact surface arranged to slidably engage the internal surface of a perforated casing in use and impart vibrations to said casing when the apparatus is rotated in the wellbore and the impact surface slides along said internal surface of said perforated casing; and at least one nozzle arranged to direct a jet of pressurised fluid from the internal chamber out of the apparatus. 